Summary of the ISM Measurements Program at Ohio University

Described is a summary of the FAA's concerns, program objectives and test procedures for a measurements program to determine the field strengths that exist around and above Industrial, Scientific, and Medical Equipment (ISM) that operate in the 27 MHz frequency band. The concerns involve determining what field strengths can exist in the vicinity of ISM equipment (principally 4th harmonic energy) that could cause interference to the Instrument Landing System (ILS) Localizer aeronautical service that operates in the 108-112 MHz frequency range. Measurements are to be made to determine the ground-to-ground and ground-to-air propagation mechanisms that exist at several installations of ISM equipment. Measurements will be made spacially above the ISM equipment using an aircraft. Analysis and recommendations will be made to indicate the validity of current measure­ ment techniques used in the certification of ISM equipment regarding RFI to aeronautical radio services. The FAA has contracted with the Ohio University Avionics Engineering Center to investi­ gate radiation levels and measurement techniques of Industrial, Scientific, and Medical (ISM)equipment in support of the development of radiation limits which would allow both aeronautical and ISM opera­ tions to perform without hazard or undue restraint. The results of the investigation will additionally assist the FCC as well as ICAO in resolving the disparity between the International vs. National limits. In particular, the FAA will provide the FCC with data regarding the allowable radiation within aeronautical bands from ISM equipment. This paper will outline the FAA's concerns, the contract objectives, and procedures in order to obtain data on radiated signal strengths from ISM equipment for future recommendations to the FCC. The FAA is concerned with the measurement techniques used during the FCC type-acceptance certification of ISM equipment as it relates to EMI in the bands allocated to aeronautical services. Traditionally, the FAA has assumed that a level of 10 micro-volts/meter (pv/m) at the receiver was a good approximation of the receiver susceptibility to radiated EMI. The current FCC limit would allow an ISM signal level capable of causing interference to exist at altitudes up to approximately 5000 ft., assuming the ground-to-air propagation is equal to ground-to-ground propagation. In the FCC and CISPR standards, a propagation mechanism proportional to 1/d or l/d^ was assumed. However, over irregular terrain a propagation mechanism of l/d3 or 1/d^ could be possible which would allow much higher signal levels near the ISM equipment when the measurements are made at 1 mile than when made near the equipment. The ground-to-air propagation mechanism, which is much closer to the l/d2, would not attenuate the radiated signal sufficiently to prevent interference. Therefore, measurements made on the ground at 1 mile may not be indicative of the signals that may exist above the ISM equipment. This would make the standard used for these measurements no longer acceptable for determining if interference to airborne receivers is possible. Also, the ground-based interference measurements cannot determine if there are any major lobes of radiation in the vertical direction. The principle frequency range under consider­ ation of the FAA is the 108-112 MHz ILS localizer aeronautical service. Very few ISM equipment operate in this band but the 4th harmonic of the 27,12 MHz +.6Z (26.96-27.28 MHz.) allocation produces 4th harmonic energy in the 107.83109.13 MHz. band, part of the ILS localizer fre­ quency range. The majority of the complaints received by the FAA are due to the ISM equipment that cause interference in this frequency range. This work, to be completed by the study, will concentrate on the propagation of vertical lobes of radiation from ISM equipment in the 27 MHz fre­ quency range and compare the ability of the FCC and CISPR measurement techniques to predict ISM signal levels in the air above the equipment. The major objectives of the work are the following: , To determine the ground-to-ground propagation mechanisms in effect at test sites and actual ISM equipment installations at the fundamental and 4th harmonic of the operating frequency of typical ISM equipment.